A large portion of the highway infrastructure is in need of repair. Studies indicate that nearly a quarter of the half million highway bridges in the United States have been classified as structurally deficient. Each year, on average, between 150 and 200 spans suffer partial or complete collapse. Current estimates for repairing deficient bridges are of the orders of $100 billion.
The potentially catastrophic consequences of fatigue cracking can be avoided by the early detection of fatigue cracks. Additionally, early detection of cracks can significantly reduce the cost of repairs.
A national mandate requires that bridges be inspected at least every two (2) years. The most frequently used method of inspecting bridge components for fatigue damage has been the elementary method of visual inspection. In order for the inspection to be of any value, the inspectors must find flaws early and accurately judge which ones need immediate repair, and which ones can wait. Although an inspector will typically examine the entire structure, a high percentage of cracks occur in common or "critical" locations. The most revealing sign of a crack is the existence of rust, oxide film and powder. However, since rust does not always appear immediately after a crack is formed, cracks may go undetected during visual inspection.
Once a crack is observed or suspected, the structure is further tested to determine the extent or severity of the damage. A number of techniques are currently used to confirm the existence of a crack. One well-known technique is the application of a dye-penetrant to the crack in question. A liquid penetrant is applied evenly over the surface being tested and allowed to enter open discontinuities. The excess surface penetrant is removed by wiping and the surface is dried. A developer is then applied, drawing the penetrant out of the discontinuity, staining the developer.
Another widely used technique is magnetic particle testing. This test method is used to detect cracks in steel by applying a magnetic field through the surface with a permanent or electromagnet. The specimen is then sprayed with an ink containing fine magnetic dust. The difference in flux density at a crack causes the particles to be attracted to the crack, which makes the crack visible.
Another method, ultrasonic testing, involves the transmission of ultrasonic pulses by piezoelectric transducers through a material. Changes in the amplitude of the received signal indicates the presence of a crack or flaw.
Still another technique, Eddy Current Imaging, measures changes in electrical impedance, produced in a material by an induction coil. A flaw changes the detectable current.
Acoustic emission testing involves the monitoring of transient waves resulting from energy releases due to crack growth.
X-ray and penetrating radiation methods are also used for flaw detection in materials.
Brittle Lacquer coatings have long been recognized as a means for evidencing the existence of strain in a material. These coatings crack in response to the substrate. In order to be of quantitative value, the coating must be used in a controlled environment, and applied in a precise and uniform manner. The lacquer coatings are also limited to work that can be closely observed so that the cracks in the lacquer can be seen. These brittle coatings are of little use for corrosion protection.
Photoelastic coatings are another known crack detection technique. Photoelastic materials, when subject to a stress or strain field, exhibit bifringence, which is seen as a fringe pattern when viewed through a polariscope.
The aforementioned prior art detection techniques require the initial step of observation of a crack by a trained inspector. Additionally, many of these techniques require complex machinery, and/or cumbersome and expensive physical removal of the structural member in question on order to confirm the existence of, or evaluate the extent, of the crack.
Visual inspection of bridge components on site, and at susceptible locations of the structure, remains the first line of action. Thus, there is a current need to improve an inspector's ability to detect cracks during an initial visual inspection. Early detection will result in savings of time and money. Making inspections more accurate will reduce the number of inspections needed, and will minimize repair costs.
Among the objects of the present invention is to provide a method for aiding in the early detection of cracks in a structure. Another object of the present invention is to provide an improved protective coating for structures. A still further object of the invention is to provide a self-activating crack indication system visible to observers with minimal training. A still further object of the present invention is to provide a non-destructive crack indication technique.